Method of making a positive temperature coefficient device

Abstract

A positive temperature coefficient ("PTC") device body is formed by compression molding of an organic PTC composition consisting of a crystalline polymer with conductive particles dispersed therein. The body is sandwiched between electrodes to which terminals are attached by spot welding or soldering. The PTC device body is aged at normal pressure and at a temperature higher than the peak resistance temperature, that is, the temperature at which its resistance is a maximum, and lower than 250° C. The resistance of the PTC device does not change either before or after exposure to high temperatures, that is, temperatures substantially greater than 250° C., and the PTC characteristics of the device do not decrease despite aging. Therefore PTC devices made according to the present invention can be surface-mounted.

What is claimed is: 
     
       1. A method of making a PTC device, which comprises: dispersing a plurality of conductive particles within an organic composition comprising at least one crystalline polymer having positive temperature coefficient characteristics;   forming a body by heating and pressure-molding said organic composition;   aging said body under normal pressure at a temperature no lower than a temperature where said body reaches its highest value of resistance;   attaching an upper electrode and a lower electrode to said body; and   connecting at least one terminal to each of said upper electrode and said lower electrode.   
     
     
       2. A method of making a PTC device as in claim 1, wherein the step of connecting includes spot-welding. 
     
     
       3. A method of making a PTC device as in claim 1, wherein the step of connecting includes soldering. 
     
     
       4. A method of making a PTC device as in claim 1, wherein said at least one crystalline polymer includes polyethylene. 
     
     
       5. A method of making a PTC device as in claim 1, wherein the step of dispersing includes heating thermal black in a nitrogen atmosphere at 1000° C. for a period in the range from 13 to 18 hours to form said plurality of conductive particles. 
     
     
       6. A method of making a PTC device as in claim 1, wherein the step of dispersing includes crosslinking said at least one crystalline polymer. 
     
     
       7. A method of making a PTC device as in claim 6, wherein the step of crosslinking includes crosslinking by radiation. 
     
     
       8. A method of making a PTC device as in claim 1, wherein the step of attaching includes attaching a metal foil to serve as said upper electrode and said lower electrode. 
     
     
       9. A method of making a PTC device as in claim 1, wherein the step of aging said body includes heating at a temperature no higher than 250° C. 
     
     
       10. A method of making a PTC device, which comprises: dispersing a plurality of conductive particles within an organic composition comprising at least one crystalline polymer having positive temperature coefficient characteristics;   forming a body by heating and pressure-molding said organic composition;   aging said body under normal pressure at a temperature no lower than a temperature where said body reaches its highest value of resistance and no higher than 250° C;   attaching a metal foil to said body to serve as an upper and a lower electrode; and   connecting at least one terminal to each of said upper electrode and said lower electrode by one of spot-welding and soldering.   
     
     
       11. A method of making a PTC device as in claim 10, wherein said at least one crystalline polymer includes polyethylene.